The general principle of a sensor of this type is illustrated in FIG. 1. The proximity sensor 100 comprises radiation source 102 and radiation detector 104. The radiation source 102 emits radiation which is reflected from an object 106 and picked up by detector 104. The detector 104 may also be provided with other circuitry provided as part of the detector 104 or associated therewith, which analyzes the output from the detector 104 for a proximity sensing calculation.
The proximity sensor 100 may also be provided with a reference detector 108. This is arranged close to the radiation source 102, such that it does not receive any parasitic (unwanted) light that is reflected from the object 106. In order to switch the radiation sensor to emit radiation, an electrical signal must be applied. This is achieved by the switching of control circuitry connected to the radiation source, connected via bond pads to a substrate such as a printed circuit board (PCB). The timing of the control signal is well known and measureable. However, there is a delay between the control signal being applied and the actual output of the radiation from the source 102. This delay can cause error in a time of flight based distance measurement. The reference detector 108 detects radiation generated when the actual output of radiation from the source 102 occurs, so that the offset between the control signal being applied and the actual emission of the radiation can be measured and taken account of in a time of flight calculation.
In order to provide an effective measurement of the time of radiation emission, the reference detector 108 must be positioned in the direct path of the main beam of the radiation detector, or part of the main beam must be reflected back on to the reference detector 108. Either of these options reduces the amount of radiation that is available for transmission and subsequent reflection from an object, thus reducing the accuracy and/or the effective range of the proximity sensor. It is therefore desired to improve the accuracy of the operation of the reference detector.
FIG. 2 shows a plan view of the sensor 100 showing the radiator 102, the detector 104 and reference detector 108. As can be seen here, the reference detector 108 and the radiation source 102 both take up space on the die. A typical module size of a proximity sensor package might for example comprise a length of 5-6 mm and a breadth of 3-3.5 mm. The radiation source may take up 1-2 mm of the package length. It is to be appreciated that these exemplary dimensions are mentioned for the purposes of illustration only. Actual dimensions could vary. In any event, the reference detector 108 is also provided as a separate component taking up further space on the die. It is desired to use proximity sensors of this type in various applications that require the sensor to be as small as possible; for example, for incorporation into mobile telephones and other portable electronic devices. It is therefore highly desirable to reduce the package size of a proximity sensor of the type that comprises a radiation source, a detector and a reference array.